Superconductivity is a property of materials where the Ohmic resistance vanishes when the material is cooled below a critical temperature TC which depends, e.g., on the material. It plays a particular important role for applications involving very high current densities as, for example, for magnets used to generate strong magnetic fields. Such strong magnets are needed, for example, in magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), or in accelerators to guide a beam of charged particles.
To maintain the superconducting state, it is not only needed to keep the temperature below the critical value. In addition, a possible external magnetic field and a current flowing through the magnet have to be below respective critical values to maintain the superconducting state. Therefore, within a configuration space spanned by the temperature, the applied magnetic field and the current density, the transition between the superconducting/normal-conducting states is associated with a surface and traversing this surface will result in a phase transition where the superconducting property is lost.
It may happen that a particular portion of the superconductor undergoes a transition into the normal conducting state (e.g. when the temperature, the current density, and/or the applied magnetic field suddenly change locally). This situation is commonly referenced to as a quench. If a quench occurs in the coil of a superconducting magnet, the normal-conducting portion of the coil heats up due to localized Joule heating and may damage the superconductor in the case the temperature in the hotspot is increased too much. In order to prevent these severe consequences, protection systems are needed that detect a quench and, when a quench is detected, enable to quickly extract the energy stored in the magnet or to enhance the propagation of the normal zone (normal-conducting portion of the conductor) thus increasing the volume where the heat is deposited and decreasing the hotspot temperature.
In such protection systems two aspects should be considered: (i) to limit the hotspot temperature in the superconductor and (ii) to prevent high voltages to ground and across the coil. The respective target limits depend on the concrete application. For accelerator magnets, for example, a temperature target value may be 400 K, but the temperature should preferably be below 300 K (or below 100 K) to limit mechanical stresses in the coils. As for the prevention of high voltages, the voltage to ground will depend on the insulation design, for example, accelerator magnets at CERN use the following: less than 5 kV and from turn to turn of coils below 500 V and from layer to layer equal or less than 2 kV. Also these values vary over a large range depending on the application. Magnets used in accelerator applications may require a current up to tens of kA which has to be discharged in a few hundred milliseconds to limit the chance of damaging the system.
Conventional protection systems are, for example, energy extraction systems and quench heaters. The energy extraction system features a switch mechanism which is opened in the case of a quench being detected; the magnet current is thus forced through an energy extraction resistor, and is discharged exponentially with a time constant L/R, where R is the external resistance and L the inductance of the magnet. The total time needed to extract the current from detection, verification, power supply switching-off, and, finally, activation of the protection system is crucial for the magnet protection. The size of an extraction resistor, and thus the discharging time constant of the system, is limited by the maximum voltage allowed across it, U=R·I. For some magnets, the ratio of inductance to resistance is such that it cannot be protected by an extraction resistor as the voltage gets too high to extract the current and protect the hotspot. In such a case, the magnet can be protected by alternative systems aimed at enhancing the quench propagation velocity and thus depositing the heat more homogeneously over the magnet coil.
Examples of such systems are the quench heaters, i.e. strips of normal-conducting metal glued to the insulation layers on the outside of the superconductor. When a quench is detected, the magnet is protected by applying a current throughout the quench heaters. The heat developed by Joule heating propagates, for example, in the adjacent cable and turns it to normal state. The aim is to induce a quench in the largest possible volume of the coil so that the energy is spread over the coil and the hotspot temperature is kept low. Nevertheless, quench heaters rely on the thermal diffusion across insulation layers, a process inherently inefficient, and may be significantly affected by highly efficient Helium cooling.
FIG. 5 depicts an example for an energy extraction resistor 410 protecting a coil 420 of a superconducting magnet arranged within a cryogenic area 430. In addition, a current source 440 and a switch 450 are connected to the coil 420 such that the switch 450 connects either the current source 440 in series to the coil 420 or, if quench has been detected, connects the resistor 410 in series to the coil 420. Therefore, if a quench is detected within the coil 420, the switch 450 disconnects the current source 440 and guides the current through the resistor 410 so that the resulting LR-circuit provides the exponential decrease of the current, thereby removing a large fraction of the energy stored in the magnet to avoid degrading of the magnet. In this circuitry for a given inductance L higher resistance R leads to a faster decay of the current, however, with the drawback that the voltage over the coil 420 gets higher.
On the other hand, the internal quench heaters rely on the heating of the superconducting magnet itself, thereby initiating also the transition in some remaining parts of the superconductor.
FIG. 6 depicts an example of such quench heaters acting on four coils 421, 422, 423, and 424. In this example, for each coil 421, 422, . . . a respective quench heater resistance 471, 472, . . . is provided inside the cryogenic area 430 to heat the respective coil in case a quench is detected. In addition to the quench heaters, each of these coils is also connected in parallel to a respective shunt diode 461, 462, 463 and 464. If a quench occurs in one of the coils 421, 422, . . . the voltage drop over the respective coil will increase, thereby allowing a current to flow over the shunt diode (because the voltage is above the forward voltage drop of the diode) and thus by-passing the current of the respective coil. Therefore, when the magnet quenches the voltage across the respective diode 461, 462, . . . opens the diode to connect the resistor and protects the magnet section by section. The plurality of coils 421 to 424 are again arranged within the cryogenic region 430 and are connected in series to a current source 440.
However, quench heaters transfer heat from the heater to the coil through insulation layers, a process which may be too slow to quench in time a fraction of magnet large enough to protect the magnet. A further problem is that the inductance of the magnet may be too large with respect to the available coil resistance which is a design problem and cannot easily be circumvented.
Quench heaters are a weak point in the magnet design because they need to be embedded in the superconducting coils separated by an insulation layer a few micrometers thick. Their presence increases the chances of electrical break-down, which is one of the most frequent causes of damage in a superconducting magnet. If the insulation between quench heaters and magnet coil is faulty, the quench heaters cannot be easily repaired and the whole magnet has to be replaced.
They are not well suited for the protection of particular superconducting magnets (e.g. based on an Nb3Sn manufacturing process). In addition, quench heaters can be easily placed only at particular locations on the magnet, typically in the outer region, and are therefore not appropriate to transfer heat in the whole coil and initiate a fast global quench in the magnet. Furthermore, tests have shown that further developments are needed to stop the heat from the quench heater being diverted into the helium bath away from the coil.
A further prior art solution uses a discharge pulse into the magnet to develop a high current pulse and so pushing the current above the critical value. The applied current pulse to the magnet also changes the magnetic field inside the superconductors, thus generating inter-filament and inter-strand coupling losses which deposit heat in the superconductor and increase the local temperature.
However, the new-generation high-energy superconducting accelerator magnets feature highly-efficient helium cooling and higher temperature margins. It became apparent, as a result of testing, that the standard quench protection was not well suited for the protection of such magnets.
Therefore, it is the object of the present invention to provide an apparatus and a method for quenching at least a part of the superconductor which is able to quench efficiently large portions of the magnet within a short period of time.